Blackbody Radiation ; Wien's Displacement Law

"Wilhelm Wien was born on January 13, 1864 at Fischhausen, in East Prussia. He was the son of the landowner Carl Wien, and seemed destined for the life of a gentleman farmer, but an economic crisis and his own secret sense of vocation led him to University studies. When in 1866 his parents moved to Drachstein, in the Rastenburg district of East Prussia, Wien went to school in 1879 first at Rastenburg and later, from 1880 till 1882, at the City School at Heidelberg. After leaving school he went, in 1882, to the University of Göttingen to study mathematics and the natural sciences and in the same year also to the University of Berlin. From 1883 until 1885 he worked in the laboratory of Hermann von Helmholtz and in 1886 he took his doctorate with a thesis on his experiments on the diffraction of light on sections of metals and on the influence of materials on the colour of refracted light."

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When any material is heated it emits electromagnetic radiation.For example if a piece of metal is heated it will emit electromagnetic  radiation mostly in the infra-red region.Now, if we continue to increase the temperature and reach 1000 degree Centigrade we will observe that the metal is now glowing with reddish color, that means the emitted em radiation has just entered visible region.The wave length is shorter than the previous case.If we continue to increase the temperature we will see that the metal is glowing with blue color.Thus we can say that the nature of the radiation depends upon the temperature of the material.

Black body:   A heated body not only emits radiation but also absorbs a fraction of the radiation falling on it.

'If a body adsorbs all the radiant energy falling on it then its absorptive power is unity. Such a body is called a black body '-S.N.Ghosal.Introductory Quantum Mechanics. 

An ideal black body is realized as a hollow enclosure with a very small orifice (O).Its inner surface is coated with lamp-back.The radiation entering the cavity is partly absorbed and pertly reflected and this reflected beam is again absorbed by the inner surface as there is very little chance that the reflected light will come out of the small orifice.So the reflection and absorption continues until 100% of the incident beam is absorbed.so the cavity behaves like a black body.

The inner walls of heated cavity also emits radiation and this radiation also has the characteristics of black body radiation.This radiation can be analyzed by infrared spectrometer.  
The result shows that radiation is continuous spectrum as shown below 

     As seen from the graph that as temperature increases Eλ also increases but λm decreases that means wavelength is decreasing with increase in temperature. 

 

So the following empirical relationship known as Wien's displacement law

    it is possible to deduce a thermodynamic law relating power radiated  per unit area.This is known as Stefan-Boltzmann law(1879)